<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>
    div {
      width: 100px;
      height: 100px;
      background-color: red;
    }
    #ddd {
      font-size: 50px;
      border: 5px solid blue;
      border-radius: 15px;
      box-shadow: 5px 5px 5px 5px #000;
      background-image: linear-gradient(purple,blue,pink);
    }
    .bt {
      background-color: yellow;
    }
    #d2:hover {
      background-color: #000;
    }
  </style>
</head>
<body>
<div style="color: blue;">
  1
</div>
<div id="d2">2</div>
<div id="ddd">3</div>
<hr>
<h3 class="bt">标题1</h3>
<h3 class="bt">标题2</h3>
<h3>标题3</h3>
<h3>标题4</h3>

</body>
</html>